The invention relates generally to atmospheric pressure ion sources, such as electrospray ion sources (ESI), chemical ionization ion sources (APCI) and photo-ionization ion sources (APPI), having novel exhaust systems. Some applications require ionization of analytes that are contained in a liquid carrier medium, such as solvent. In a liquid chromatography—mass spectrometry (LC/MS) interface, for example, the eluent from an LC column is introduced into an ionization chamber that is maintained at, or close to, atmospheric pressure. Basically, the three above-indicated ionization mechanisms have found wide-spread application with atmospheric pressure ion sources.
The eluent can be ionized by electrospray where a high voltage difference, such as ranging from one to eight kilovolts, is generated between a conduit delivering the liquid eluent and an appropriate counter-electrode in order that charged droplets are generated. A nebulizer gas can be used in order to shear the droplets and further reduce their size (pneumatically-assisted electrospray). Still other desolvation or drying gases can be added with temperature above ambient, such as several hundred degrees centigrade, in order to promote solvent evaporation. Details of the ESI technique have been discussed in the literature; see, for instance, a recent review article by S. Banerjee and S. Mazumdar “Electrospray Ionization Mass Spectrometry: A Technique to Access the Information beyond the Molecular Weight of the Analyte”, International Journal of Analytical Chemistry, Volume 2012, Article ID 282574, 40 pages, doi:10.1155/2012/282574.
In an APCI ion source the eluent from the LC is introduced and nebulized in a heater zone in order to vaporize the liquid. The eluent in the gas phase is ionized via primary and secondary charge transfer reactions with reagent ions originating from a reagent ion source gas that is ionized by a corona discharge. A variety of means, such as introducing a heated gas, can be used to transfer the energy necessary for vaporization as is known in the art.
Instead of charge transfer reactions, photons may be used for ionizing the sprayed and vaporized eluent in a photo-ionization process. APPI has been described, for example, in an early report by Robb et al., Analytical Chemistry, Vol. 72, No. 15, Aug. 1, 2000 3653-3659.
A portion of the ionized eluent in the form of gas-phase ions and tiny charged droplets is sampled into the inlet of the mass spectrometer while the remains of the spray droplets and the gases assisting in the spraying and evaporation need to be removed from the source housing to avoid recirculation and possible memory effect responses from the mass spectrometer. The exhaust port typically is an opening at the bottom of the source housing, which allows evacuation of unevaporated droplets, residual spray mist, solvent vapor and gas from the source chamber. Usually such a port is located opposite to the spray probe that delivers the liquid eluent and has a cross section area that generally matches the dimensions of the spray cone at its entrance, preferably with a slight oversize. The exhaust port is connected to an exhaust tube which further carries away the waste out of the ion source chamber. Ideally, such a tube is co-axial with a general spray direction and should extend to an infinite length without any change in direction to establish the most favorable flow conditions and to be most effective in avoiding a back flash flow that returns to the ion source chamber. In reality, for practical reasons, however, the exhaust tube needs to be bent at some point to change the exhaust flow direction.
U.S. Pat. No. 7,145,138 B1 to Thakur teaches that a change in flow direction in the exhaust tube can be used to prevent back flash of liquid into the source. However, practice shows that the results achievable with this design are not quite satisfactory.
US application 2011/0068263 A1 to Wouters et al. presents an ion source where a tip of the spray probe is located in a continuous flow guide. In the spray direction, a cross-sectional area that defines a first portion of an internal volume of the flow guide initially decreases in a convergent-like manner and thereafter increases in a divergent-like manner towards an exit opening of the source housing. The aim is to provide for unidirectional flow past a sampling orifice of a mass spectrometer inlet to prevent recirculation of waste gas and solvent. Such a design requires significant modification of the source housing design and is therefore generally not desired.
U.S. Pat. No. 6,614,017 B2 to Waki teaches a droplet or liquid collector in a forward spray direction as to avoid bouncing back of droplets into an ion sampling region. This teaching may be adequate for liquid droplets but largely fails to address the adverse effects of excess gas-phase solvent recirculating in the source housing, for example.
U.S. Pat. No. 6,459,081 B2 to Kato presents an API mass spectrometer that is supposed to prevent effects of nonvolatile salts on the mass analysis without deteriorating the vacuum condition of the mass analysis portion. Essentially, crystals of nonvolatile salts precipitated on certain surfaces in a spray chamber are washed away with a washing solution, such as water.
Hence, there is still a need for an exhaust system to be operated with an atmospheric pressure ionization source that reduces the risk of residual spray mist and waste gas recirculation in an ionization chamber.